1. Verify that there's no debris in the headphone port.
2. Connect and then disconnect the headphones.
3. Verify that the headphone connector is pushed in all the way, so that the headphone's plug shell is flush with the headphone port, like this:
4. Try resetting the iPod:
- Toggle the Hold switch on and off.
- Press and hold the menu and center buttons simultaneously until the Apple logo appears.
- If you need assistance with resetting your iPod, see this article.
5. Try another set of Apple headphones.
6. If you have tried all the previous steps and you still hear sound coming from the speaker when the headphones are connected, your iPod may need service.